The usual convention is that square brackets group things that are optional. You have several levels of nested square brackets to interpret. The square brackets group things like parentheses in math expressions - everything within a set of square brackets must be considered as a single unit.
That entire construct is in square brackets - that means that whole "chunk" is optional - it can be there, or not, as a single unit.
Let's assume that we want it to be there, so let's strip away the outermost level of square brackets, and then we're left with:
Which means, if anything at all from the original construct is present, it must start with
-n, followed by
number-to-stat. Note that
number-to-stat is not optional in this case - if
-n is there, then it must be followed by a
number-to-stat. Everything following that is within yet another group of square brackets, so it too, as a single unit, is optional.
If anything does follow the
number-to-stat, it must be in this form:
Which means that the next part of the expression must be
:max-size. That can optionally be followed by a
:min-size - or not, as you like... But that by itself is in another grouping of square brackets, so it is a single unit, independent of the rest.
Whether or not
:min-size is present, you could also then optionally append
:num-directories, and if you do, you could then optionally append
:chunk-size to that. If you didn't put a
:num-directories, you can't put a
:chunk-size, because they're together within a single group of square brackets.
To answer your direct question, if you use the
-n option at all, you can't omit the
number-to-stat, that is going to be required. You can omit
:max-size, but then if you do so, you can't provide
:min-size (I know you said you didn't want to anyway). After that, you can give
:num-directories, and you can omit the
:chunk-size. And, if course, if you don't provide the
-n, then you can't provide any of the rest of it.
And, of course, this all is contingent upon the assumption that the example you gave uses the square brackets in the way they're typically used in documentation.